(a) Field of the Invention
The present invention relates to a semiconductor laser device having a lower threshold current.
(b) Description of the Related Art
Semiconductor laser devices operating at a wavelength band between 1.2 μm and 1.3 μm now attract larger attention for use as light sources in optical communication scriber lines.
Although a GaInAsP semiconductor laser device formed on an InP substrate is conventionally developed as a semiconductor laser device having an emission wavelength of 1.3 μm band, it has been a problem that the semiconductor materials used in the GaInAsP laser device has a poor characteristic temperature of the threshold current as low as 50K to 70K, which degrades the temperature characteristics of the laser device.
In order to dispose a semiconductor laser device in each of households as a light source, it is desired that the semiconductor laser module have a longer wavelength band and a superior temperature characteristic without using a cooling device, and also can be manufactured at a lower cost.
Thus, a variety of semiconductor laser devices having improved temperature characteristics and longer emission wavelengths have been developed. It is reported in “Jpn.J.Appl.Phys., vol.35(1996)”, pp.1273-1275 by M. Kondow et al. that one of the semiconductor laser devices thus developed may have a temperature characteristic as high as about 180K. Also it is assured therein by an experiments that a prototype semiconductor laser device achieved a higher temperature characteristic around 130K to 270K.
Moreover, it is also reported by F. Koyama et al., in IEEE Photon. Technol. Lett., vol.12(2000) pp.125-127 that a characteristic temperature as high as around 140K-170K was realized by using a high-strained GaInAs semiconductor laser device operating at an emission wavelength band of 1.2 μm. It is to be noted that since the commercial single mode (SM) optical fiber has a cutoff for fundamental mode at 1.2 μm wavelength, the semiconductor laser device is expected for use as a light source in a local area network (LAN).
The conventional high-strained semiconductor laser device as described above suffers from the higher amount of strain and a poor mixing capability of the constituent element, nitrogen (N), into the V-group elements due to the fact that nitrogen has a smaller radius of atoms. This involves an undesirable three-dimensional growth and a large number of crystal defects unless the growth temperature is lowered to a minimum, which however degrades the optical quality for the crystal structure.
It is also noted that the high strained layer is difficult to grow with involving a longer-distance migration by using a molecular beam epitaxy (MBE) generally used for growing semiconductor layers.